This application is based on Japanese Patent Application Nos. 11-104584, 11-104585, 2000-35455, and 2000-35456, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The invention relates to an apparatus, a method and a computer program product for processing image data, in particular, to the edge detection.
2. Description of Related Art
Printed materials generally have halftone images, i.e., images described by numerous halftone dots. When images of printed matters or documents are captured by scanning and outputted by a printer, Moirxc3xa9 patterns appear depending on the relation between spatial frequencies of halftone dots and scanning cycles or dither pattern cycles during their dither processes.
A typical conventional digital copying machine detects a halftone dot region where halftone dots exist, for applying the smoothing process to the halftone dot region to make the edges of the dots less conspicuous. The copying machine also applies the edge enhancement process to character elements to make the character elements reproduced sharper.
However, the method does not function properly when character elements exist in the halftone dot region. For example, the reproducibility of the character elements deteriorates if the smoothing process is applied to the halftone dot region. On the other hand, minute edges of halftone dots will be enhanced causing the Moirxc3xa9 effect and deterioration of picture quality if the edge enhancement process is applied to the same region.
There is another method in which only the edges of character elements are detected and the enhancement process is applied to the detected edges in order to reproduce the character elements more sharply. The edge detection is based on the lightness gradient or the density gradient.
The lightness data used for the edge detection is calculated by adding the RGB-image data at a fixed rate. More specifically, the lightness data V is calculated by the following formula, where coefficients k1, k2 and K3 are constants:
V=k1xc3x97R+k2xc3x97G+k3xc3x97B
However, the coefficient k2 or the weight of the G-image data is set heavier than others in order to correspond to the human visual characteristics. Therefore, the lightness gradient between the background and the character elements will be detected smaller than usual if black character elements exist against a red background or blue character elements exist against a white background. Accordingly, the edge detection accuracy deteriorates when character elements exist against a background of a certain color.
In addition, Publication of Unexamined Japanese Patent Application No. 6-38054 discloses a method of the edge detection based on the density gradient. The density data used in the method is the image data of magenta which is the closest to the human visual characteristics among the CMY-image data obtained by logarithmic conversion of the RGB-image data. Consequently, a problem of difficulty in detecting edges may occur if black character elements exist against a magenta background. In other words, the edge detection accuracy deteriorates when character elements exist against a background of a certain color as well.
It is an object of the present invention to provide an image processing apparatus including a minimum selector, a maximum selector, and a detector for processing image data having a plurality of color components expressing the image. The minimum selector selects an image data of a minimum value color component from the image data having a plurality of color components. The maximum selector selects an image data of a maximum value color component from the image data having a plurality of color components. The detector detects an edge existing in the image based on the image data of two color components selected by the minimum selector and the maximum selector.
A further object of the invention is to provide an image processing method including a minimum value data selecting step, a maximum value data selecting step, and a detecting step for detecting an edge segment existing in an image based on image data having a plurality of color components expressing the image. The minimum value data selecting step is for selecting an image data of a minimum value color component from the image data having a plurality of color components. The maximum value data selecting step is for selecting an image data of a maximum value color component from the image data having a plurality of color components. The detecting step is for detecting an edge existing in the image based on the image data of two color components selected by the minimum value data selecting step and the maximum value data selecting step.
Still a further object of the invention is to provide a computer program product for executing a minimum value data selecting step, a maximum value data selecting step, and a detecting step to detect an edge segment existing in an image based on image data having a plurality of color components expressing the image. The minimum value data selecting step is for selecting an image data of a minimum value color component from the image data having a plurality of color components. The maximum value data selecting step is for selecting an image data of a maximum value color component from the image data having a plurality of color components. The detecting step is for detecting an edge existing in the image based on the image data of two color components selected by the minimum value data selecting step and the maximum value data selecting step.
Another object of the invention is to provide an image processing apparatus including a lightness calculator, a saturation calculator, and a detector for processing image data expressing an image. The lightness calculator calculates a lightness component from the image data. The saturation calculator calculates a saturation component from the image data. The detector detects an edge existing in the image based on the lightness component and the saturation component calculated by the lightness calculator and the saturation calculator.
A further object of the invention is to provide an image processing method including a lightness calculating step, a saturation calculating step, and a detecting step for detecting an edge segment existing in an image based on image data expressing the image. The lightness calculating step is for calculating a lightness component from the image data. The saturation calculating step is for calculating a saturation component from the image data. The detecting step is for detecting an edge existing in the image based on the lightness component and the saturation component calculated by the lightness calculating step and the saturation calculating step.
Still a further object of the invention is to provide a computer program product for executing a lightness calculating step, a saturation calculating step, and a detecting step to detect an edge segment existing in an image based on image data expressing the image. The lightness calculating step is for calculating a lightness component from the image data. The saturation calculating step is for calculating a saturation component from the image data. The detecting step is for detecting an edge existing in the image based on the lightness component and the saturation component calculated by the lightness calculating step and the saturation calculating step.
A further object of the invention is to provide an image processing apparatus including a saturation calculator and a detector for processing image data expressing an image. The saturation calculator calculates a saturation component from the image data, and the detector detects an edge existing in the image based on the saturation component of the image data calculated by the saturation calculator.
The objects, characteristics, and advantages of this invention other than those set forth above will become apparent from the following detailed description of the preferred embodiments, which refers to the annexed drawings.